1. Field of the Invention
This invention relates to a pressure switch and, more specifically, to a pressure responsive switch for mounting on a printed wiring board or an insert molded lead frame which is capable of operating in the normally closed condition.
2. Brief Description of the Prior Art
It is relatively standard in the automotive art to control various functions by means of microprocessor based control units to obtain performance improvements.
One such application has included the operation of the transmission system by integrating engine and transmission control. Such operation requires that the transmission control be compatible with the engine control module (ECM) and be electronically accessible with inputs and outputs. One such prior art approach has utilized solenoid valves to effect gear shifting using pressure switches in the solenoid valve assembly as a way to confirm that solenoid valve actuation and deactuation has occurred responsive to pressure change in the hydraulic fluid. This pressure change is sensed using conventional snap acting pressure responsive switches which close or open electrical circuits on the occurrence of selected pressure levels. A problem with switches of this type is that snap acting switches have a lower life expectancy than is desired.
In U.S. Pat. No. 4,758,695 there is disclosed an attempt to minimize this problem wherein a control system is provided where a metallic diaphragm is used having significantly improved longevity. Such diaphragms are formed with a central dished portion having a pressure deflection relationship such that the diaphragm is relatively stiff, having a positive coefficient of pressure with increasing deflection up to and above a relatively narrow range of set points or calibrated pressures. Within the range of set points the effective spring rate of the diaphragm is relatively supple with only a small increase in pressure resulting in relatively larger travel of the center of the diaphragm. The diaphragms are also characterized in having significantly less hysteresis than conventional snap acting discs to minimize the build up of stresses in the diaphragm since these stresses serve to limit the longevity of the diaphragm. Among the embodiments disclosed are switches in which the diaphragms are formed with an annular flat berm portion which is received on an electrical contact member with an 0-ring disposed on top of the berm and biased thereagainst to form a fluid pressure seal by a tubular sleeve which communicates with an hydraulic fluid pressure source. Another embodiment provides a sleeve formed in two segments with the 0-ring sandwiched therebetween so that the sleeve itself engages the berm portion. An electrical contact rivet is placed beneath the central dished portion and connected to a suitable electrical connector. While the berm provides a convenient way to mount and seal the diaphragm, the integral interconnection between the flat berm portion and the central dished portion results in limiting the life of the diaphragm. In other embodiments, the entire diaphragm is dished and maintained on the electrical contact member by means of a thin flexible membrane which also provides a seal for the switch. However, the use of a membrane to retain the diaphragms in their respective seats limits the positioning of the stationary center contact to the low pressure side of the diaphragm (to close a circuit upon pressure increase. That is, the membrane would preclude the use of a fixed contact on the high pressure side of this diaphragm (to open a circuit upon selected pressure increase.)
A further improvement in the prior art is set forth in U.S. Pat. No. 4,861,953 by forming the entire surface of the diaphragm into a dished configuration with the center of the diaphragm having a pressure versus deflection relationship such that for increasing pressure from 0 psig up to and beyond a plateau having a range of deflections between d1 and d2, the diaphragm has a relatively stiff effective spring rate with the center deflecting between d1 and d2 at essentially the same pressure level, the diaphragm also having a relatively narrow differential between the pressure at which the center of the diaphragm deflects between d1 and d2 on increasing pressure and the pressure at which it deflects between d2 and d1 on decreasing pressure.
In application Ser. No. 07/286,726, filed December 20, 1988 and assigned to the assignee of the instant invention, switches are described comprising, in one embodiment, upper and lower housings with a snap acting member and an electrically conductive member sandwiched between the upper and lower housings. The upper housing includes an electrically insulating body with a hollow center portion which is molded around an electrically conductive member having a contact portion in the hollow center portion, the conductor extending externally of the insulating body. The snap acting member is in constant engagement with the sandwiched electrically conductive member and normally in engagement with the contact of the upper housing. When a pressure is applied which is sufficient to cause the snap acting member to snap into its second stable state, the engagement thereof with the contact in the upper housing is broken and engagement is made with the contact in the lower housing.
The switch can be provided as normally closed by removing the portion of the conductor on the lower housing which extends externally of said member. The switch can be provided as normally open by removing the portion of the conductor on the upper housing which extends externally of said member.
However, when used as a normally closed switch the structure of application Ser. No. 07/286,726 described above has certain limitations which would be desirable to overcome. One such limitation relates to the fact that when used with transmission systems contaminants in the fluid can get into the switching area of the switch causing short circuits and changes in calibration. Further, the fluids cause films to form on the disc and other contact surfaces which then necessitate higher contact force than is available in that structure to make effective electrical engagement. Another limitation relates to the high loading involved with mounting the switch to obtain an effective seal which can cause a shift in the position of the stationary contact due to the location of an O-ring which transmits force through a portion of the top housing which can bend. Yet another limitation relates to problems of dislodgement of the O-ring seal during assembly thereby causing leakage problems.
It is therefore an object of the present invention to provide a pressure responsive switch particularly useful in applications involving engine control modules (ECM) or the like which are normally closed which overcome the above noted limitations.